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Abstract
Recyclable coherent random lasers assisted by plasmonic nanoparticles in DCM-PVA thin films are studied. Four DCM-PVA films with different nanoparticles are made, and the radiation characteristics of these random lasers are studied. The results show that the emission spectrum of the DCM-PVA film with Au nanoparticle of 50 nm in diameter is optimal, and its threshold is about 6.53 µJ/pulse. Underlying mechanisms are discussed in detail. Then the DCM-PVA film with Au nanoparticles of 50 nm in diameter is detached from a glass substrate and adhered to different substrates. Coherent random lasers also occur when the sample is based on different substrates. Finally, a method of making samples recyclable is proposed, and the emission spectrum of samples as a function of cycle index is studied. The results show that recyclable coherent random lasers can be realized with this method. This study provides a new way, to the best of our knowledge, to realize recyclable coherent random lasers with low-threshold.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since Letokhov [1] theoretically proposed in 1968 that the optical feedback of stimulated emission can be provided by scatterers, random lasers as a unique class of light sources have been investigated extensively [2–10]. Light amplification mechanisms in a random system without strictly resonant cavity are multiple light scattering and stimulated emission [3,11,12]. In general, random lasers can be divided into coherent random lasers and incoherent random lasers, depending on whether the optical feedback is resonant feedback (field feedback) or nonresonant feedback (intensity feedback) [11,13]. Random lasers can be distinguished by the properties of emission spectrum. The emission spectrum of coherent random lasers has some discrete sharp peaks with sub 1 nm linewidth on the top of the emission spectrum, when pump energy exceeds its threshold [14,15]. However, the emission spectrum of incoherent random lasers is a smooth spectrum with the linewidth of a few nanometers, when pump energy exceeds its threshold [11,16].
Recently, plasmonic nanoparticles are usually used in random systems to enhance the performance of random lasers [17–24]. Plasmonic nanoparticles have a larger scattering cross section than their dielectric counterparts. This results in a stronger optical feedback in the random systems with plasmonic nanoparticles. The light is more likely to be confined in the random systems and amplified by gain [13–27]. In addition, electromagnetic field is localized and enhanced around plasmonic nanoparticles, because of the localized surface plasmon resonance (LSPR) of plasmonic nanoparticles [28,29]. These can greatly improve the fluorescent amplification efficiency of gain materials [30,31]. Therefore, both the optical feedback and gain can be enhanced by plasmonic nanoparticles. With the improvement of people's living standard and awareness of environmental protection, recyclable random lasers are desired.
In this study, a recyclable coherent random laser assisted by plasmonic nanoparticles in DCM-PVA thin films is studied. Firstly, the influence of different nanoparticles on the characteristics of random lasers is studied. And the underlying mechanisms are discussed in detail. Then, coherent random lasers based on different substrates are studied. Finally, a method of making the recyclable coherent random laser is presented.
2. Sample preparation and experimental setup
The metal nanoparticles used in our experiment are Au nanoparticles of 50 nm and 10 nm in diameter, and Ag nanoparticles of 50 nm and 10 nm in diameter. To fabricate the DCM-PVA thin films with plasmonic nanoparticles, we take the following steps. Firstly, the DCM dye is dissolved in dimethyl sulfoxide (DMSO) solution with the concentration of 0.1 wt%, and the polyvinyl alcohol (PVA) is dissolved in DMSO solution with the concentration of 10 wt%. The two solutions are mixed in a ratio of 1:10. Then, we divide the mixture into four parts. The four nanoparticles are dispersed in the four parts with the same number density of $1.5816 \times {10^{10}}/{\textrm{ml}}$, respectively. An ultrasonic oscillation process is applied to make the nanoparticles homodisperse in the mixture. A glass substrate ($2cm \times 1cm$) is ultrasonically cleaned in acetone and deionized water for 10 min, then the glass substrate is air-dried. Mixtures with nanoparticles (0.2 ml) are dropped stepwise on the glass substrate and spin-coated evenly. Finally, the samples are cured at 60°C for 4 hours to evaporate unnecessary DMSO solution. After this treatment, we obtain thin films with the thickness of about 100 µm. The number density of metal nanoparticles in thin films is about $1.5816 \times {10^{11}}/\textrm{cm}^{3}$, and the concentration of DCM dye in thin films is about 0.1 wt%. We have made four samples as follows. Sample 1: the DCM-PVA film with Au nanoparticles of 50 nm in diameter; Sample 2: the DCM-PVA film with Au nanoparticles of 10 nm in diameter; Sample 3: the DCM-PVA film with Ag nanoparticles of 50 nm in diameter; Sample 4: the DCM-PVA film with Ag nanoparticles of 10 nm in diameter.
Figure 1 shows the schematic diagram of the samples pumped by a pump stripe. Pump light is from the second harmonics of a Nd:YAG nanosecond laser (532 nm, 10 Hz, 8 ns). The pump light is focused by a cylindrical lens with focal length of 10 cm to form a pump stripe with a length of 10 mm and a width of 0.2 mm. The single-shot emission spectrum from samples is collected along the pump stripe by a fiber spectrometer with the spectral resolution of 0.13 nm.
3. Results and discussion
Figure 2(a) depicts the evolution of the emission spectrum of sample 1 as a function of the pump energy. Fi gure 2(b) shows the peak intensity and FWHM of corresponding emission spectrum dependent on the pump energy. When the pump energy is low, the emission spectrum is a broad spontaneous emission spectrum with a full width at half maximum (FWHM) of about 54 nm. However, when the pump energy exceeds a threshold, some discrete sharp peaks with the FWHM of about 0.5 nm suddenly emerge on the top of emission spectrum at the wavelength of about 620 nm. The threshold is defined by the knee point of peak intensity and FWHM of emission spectrum, as shown in Fig. 2(b). The threshold of sample 1 is about 6.53 µJ/pulse. Figure 2(c) describes the evolution of the emission spectrum of sample 2 as a function of the pump energy. Figure 2(d) shows the peak intensity and FWHM of corresponding emission spectrum dependent on the pump energy. As we can see in Fig. 2(c), although some discrete sharp peaks can also be observed on the top of emission spectrum when the pump energy is larger the threshold, the discrete sharp peaks are very weak and the threshold (17.35 µJ/pulse) is much larger than that of sample 1. These results indicate that the Au nanoparticle with the diameter of 50 nm is more effective than the Au nanoparticle with the diameter of 10 nm for the realization of coherent random lasers in the DCM-PVA thin films when the density of the nanoparticles is $1.5816 \times {10^{11}}/\textrm{cm}^{3}$. Next, we will discuss the reasons in detail.
Fig. 2. (a) and (c) The emission spectrum of sample 1 and sample 2 as a function of the pump energy. (b) and (d) The peak intensity and FWHM of corresponding emission spectrum as a function of the pump energy.
As is well-known, the characteristics of random lasers are mainly related to the scattering strength and gain strength of random systems. Therefore, we study the effect of Au nanoparticles with diameters of 50 nm and 10 nm on the scattering strength and gain strength of the system, respectively. The scattering strength of a nanoparticle can be evaluated by the scattering cross section of the nanoparticle, and the fluorescence amplification efficiency of DCM dye molecules is related to the electric filed intensity at its position. Therefore, we compare the scattering cross section and electric filed intensity distributions of different metal nanoparticles. Figure 3(a) shows the scattering cross section of Au nanoparticles with diameters of 50 nm and 10 nm, which is simulated by the finite-difference time-domain (FDTD) methods. As we can see in Fig. 3(a), the scattering cross section of one Au nanoparticle with the diameter of 50 nm is about $4.9 \times {10^{ - 12}}c{m^2}$ at the wavelength of 620 nm, which is about four orders of magnitude larger than that of one Au nanoparticle with the diameter of 10 nm. It means that the Au nanoparticle with the diameter of 50 nm is more effective than the Au nanoparticle with the diameter of 10 nm for enhancing the scattering strength of random system. Figure 3(b) and 3(c) show the electric field intensity distributions around Au nanoparticles with diameters of 50 nm and 10 nm at the wavelength of 532 nm, respectively. The electric field intensity distribution is simulated by the FDTD methods. The electric filed intensity is defined by $|{E/{E_0}} |$, where $E = \sqrt {{{|{{E_x}} |}^2} + {{|{E{}_y} |}^2} + {{|{{E_z}} |}^2}} $, ${E_0}$ is the electric field density of pump light. The electric filed is localized and enhanced near Au nanoparticles due to LSPR of Au nanoparticles. The electric filed intensity around the Au nanoparticles with the diameter of 50 nm is larger than that of Au nanoparticles with the diameter of 10 nm. The fluorescence amplification efficiency of DCM dye molecules is proportional to the electric filed intensity at its position. In other words, the stronger the electric filed intensity is, the higher the fluorescence amplification efficiency of DCM dye molecules will be. Moreover, it is worth noting that the area of the electric field enhancement of the Au nanoparticles with the diameter of 50 nm is much larger than that of the Au nanoparticles with the diameter of 10 nm. For $|{E/{E_0}} |> 1.5$, the area of the electric field enhancement of one Au nanoparticle with the diameter of 50 nm is about 3191.84 nm2, which is about 9 times larger than that (about 358.78 nm2) of one Au nanoparticle with the diameter of 10 nm. It means that the fluorescence amplification efficiency of more DCM dye molecules is enhanced by Au nanoparticles with the diameter of 50 nm.
Fig. 3. (a) The scattering strength of Au nanoparticles with diameters of 50 nm and 10 nm. (b) and (c) The electric field intensity distributions around Au nanoparticles with diameters of 50 nm and 10 nm at the wavelength of 532 nm.
The effects of the scattering strength enhancement and electric filed enhancement occur simultaneously, when adding Au nanoparticles in the DCM-PVA films. Therefore, it is difficult to study their respective roles. The scattering mean free path, which is defined by the average length between two scattering events, can be used to express the scattering strength of Au nanoparticles. It can be expressed by ${l_s} = {1 \mathord{\left/ {\vphantom {1 {\rho {\sigma_s}}}} \right.} {\rho {\sigma _s}}}$, where ${\sigma _s}$ is the scattering cross section of one nanoparticle and $\rho $ is the number density of nanoparticles. For ${\sigma _s} = 4.9 \times {10^{ - 12}}c{m^2}$ of one Au nanoparticle with the diameter of 50 nm, ${\sigma _s} = 1.34 \times {10^{ - 16}}c{m^2}$ of one Au nanoparticle with the diameter of 10 nm, and $\rho = 1.5816 \times {10^{11}}/c{m^3}$, the scattering mean free path of sample 1 and sample 2 is about 1.29 cm and $4.72 \times {10^4}$ cm respectively, which are both larger than the length of the pump stripe. It means that the scattering strength enhanced by the Au nanoparticles is weak. In such a weak scattering system, coherent random lasers only formed by scattering contributions have been rarely reported. Therefore, the role of the LSPR of Au nanoparticles is very important for forming coherent random lasers, especially for the Au nanoparticles with 10 nm in diameter. The electric field is localized and enhanced around the Au nanoparticles due to LSPR. This can greatly improve the fluorescent amplification efficiency of DCM dye molecules, which provides high optical gain for random lasers. Therefore, we can draw a conclusion that the feedback mechanism of coherent random lasers in this work comes from both the multiple light scattering by Au nanoparticles and the electric filed enhancement due to LSPR of Au nanoparticles.
To further confirm our conclusion, we studied the emission characteristics of the random lasers from sample 3 and sample 4. Figure 4(a) and 4(c) show the emission spectrum of sample 3 and sample 4 as a function of the pump energy, respectively. Figure 4(b) and 4(d) show the peak intensity and FWHM of corresponding emission spectrum as a function of the pump energy, respectively. The emission spectrum characteristics of the DCM-PVA films with Ag nanoparticles are similar to that of the DCM-PVA films with Au nanoparticles. When the pump energy exceeds a threshold, many discrete sharp peaks with the FWHM of about 0.5 nm suddenly emerge on the top of the emission spectrum at the wavelength of about 620 nm. However, the thresholds of sample 3 and sample 4 are 11.9 µJ/pulse and 23.03 µJ/pulse, respectively.
Fig. 4. (a) and (c) The emission spectrum of sample 3 and sample 4 as a function of the pump energy. (b) and (d) The peak intensity and FWHM of corresponding emission spectrum as a function of the pump energy.
Figure 5(a) shows the scattering strength of Ag nanoparticles with diameters of 50 nm and 10 nm. Figure 5(b) and 5(c) show the electric field intensity distributions around Ag nanoparticles with diameters of 50 nm and 10 nm at the wavelength of 532 nm. The scattering cross section of the Ag nanoparticle with the diameter of 50 nm is about $4.14 \times {10^{ - 12}}c{m^2}$ at the wavelength of 620 nm, which is about four orders of magnitude larger than the scattering cross section ($2.25 \times {10^{ - 16}}c{m^2}$) of the Ag nanoparticle with the diameter of 10 nm at the wavelength of 620 nm. The electric field intensity of the Ag nanoparticle with the diameter of 50 nm is slightly stronger than that of the Ag nanoparticle with the diameter of 10 nm. In addition, the area of the electric field enhancement of Ag nanoparticles with the diameter of 50 nm is much bigger than that of Ag nanoparticles with the diameter of 10 nm, as shown in Fig. 5(b) and 5(c). For $|{E/{E_0}} |> 1.5$, the area of the electric field enhancement of one Ag nanoparticle with the diameter of 50 nm is about 2248.79 nm2, which is about 8 times larger than that (about 276.44 nm2) of one Ag nanoparticle with the diameter of 10 nm. These results are consistent with our discussions and conclusions in sample 1 and sample 2. It is worth pointing out that although the electric field intensity of Au nanoparticles with the diameter of 10 nm (see Fig. 3(c)) is stronger than that of Ag nanoparticles with the diameter of 50 nm (see Fig. 5(b)), the threshold of sample 2 is higher than that of sample 3. It is mainly because the area (about 2248.79 nm2 for $|{E/{E_0}} |> 1.5$) of the electric field enhancement of one Ag nanoparticle with the diameter of 50 nm is much larger than that (about 358.78 nm2 for $|{E/{E_0}} |> 1.5$) of one Au nanoparticle with the diameter of 10 nm, and the scattering strength of Ag nanoparticles with the diameter of 50 nm is much larger than that of Au nanoparticles with the diameter of 10 nm.
Fig. 5. (a) The scattering strength of Ag nanoparticles with diameters of 50 nm and 10 nm. (b) and (c) The electric field intensity distributions around Ag nanoparticles with diameters of 50 nm and 10 nm at the wavelength of 532 nm.
The DCM-PVA film is easily detached from the glass substrate by a knife, and adhered to other substrates by a small amount of PVA DMSO solution. Figure 6(a) shows the emission spectrum of sample 1 film as a function of the pump energy, when the glass substrate is removed. Figure 6(b) shows the peak intensity and FWHM of corresponding emission spectrum as a function of the pump energy. As we can see in Fig. 6(a) and 6(b), coherent random lasers with fine discrete sharp peaks also occur in sample 1 without the substrate. The threshold is about 12.5 µJ/pulse, which is much larger than that of sample 1 with the glass substrate. This is because the light feedback caused by the reflection of glass substrate disappears in sample 1 without the substrate. In order to expand the application of samples, the sample 1 is transferred onto different items, such as bottles, rulers, books and syringes, as shown in Fig. 6(c). Figure 6(d) shows the emission spectrum of sample 1 adhering to different substrates. As we can see in Fig. 6(d), coherent random lasers are easily realized by transferring sample 1 onto different items.
Fig. 6. (a) The emission spectrum of sample 1 as a function of the pump energy, when the glass substrate is removed. (b) The peak intensity and FWHM of corresponding emission spectrum as a function of the pump energy. (c) The photographs of sample 1 adhering to bottles, rulers, books and syringes, respectively. (d) The emission spectrum of sample 1 adhering to different substrates.
Figure 7(a) shows the schematic diagram of the recyclable coherent random laser. Firstly, the DCM-PVA film with NPs is detached from glass substrate by a knife, after the emission spectrum of the sample is recorded. Next, the film is melted in 0.2 ml DMSO solutions at 60°C. And an ultrasonic oscillation process is applied to disperse nanoparticles evenly in the mixture. Then, the mixture is dropped on the glass plate and coated evenly. Finally, the samples are cured at 60°C for 4 hours to gain thin films. So far, the first cycle is completed. We run five cycles using sample 1, and the emission spectrum of sample 1 as a function of cycle index is shown in Fig. 7(b). With the increase of the cycle index, discrete sharp peaks are also observed on the top of the emission spectrum. The intensity of the discrete sharp peaks is almost unchanged, and the coherent random laser can keep good performance after several cycles. This means that random lasers formed by the DCM-PVA film with NPs can be recycled.
Fig. 7. (a) Schematic diagram of the recyclable coherent random laser. (b) The emission spectrum of sample 1 as a function of cycle index.
4. Conclusion
In conclusion, recyclable coherent random lasers formed by DCM-PVA thin films with plasmonic nanoparticles are studied. Firstly, the influence of different nanoparticles on the characteristics of random lasers is studied. The underlying mechanisms are discussed in detail. The characteristics of random lasers depend on the cooperative effect of the LSPR of metal nanoparticles and the scattering strength of random system. The results show that the threshold of the random laser based on the DCM-PVA film with Au nanoparticles of 50 nm in diameter is the lowest, and it is about 6.53 µJ/pulse. Then, the emission spectrum of sample 1 based on different substrates is studied, and coherent random lasers also occur on different substrates. Finally, the method of making recyclable random lasers is described in detail. The emission spectrum of random lasers as a function of cycle index is studied. The coherent random laser can keep good performance after several cycles. This study provides a promising platform for the realization of recyclable random lasers with low-threshold.
Funding
National Natural Science Foundation of China (11474021).
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